1. Field of the Invention
The present invention relates to a power supply device that outputs a high voltage using a piezoelectric transformer, and an image forming apparatus including the power supply device.
2. Description of the Related Art
In recent years, a color laser printer or a copying machine of an electrophotographic system as an image forming apparatus includes a plurality of photosensitive drums serving as an image bearing member, a plurality of charging rollers for charging respective photosensitive drums, and exposure devices for exposing respective charged photosensitive drums and causing latent images to be formed on each of the plurality of photosensitive drums. In addition, the color laser printer, and the copying machine include a development device for developing the formed latent images using a toner serving as a developer, and a transfer roller for transferring images developed on the photosensitive drums on recording paper.
A voltage (also referred to as a charging bias) for charging the image bearing member is applied to the charging roller described above, a voltage (also referred to as a developing bias) for developing the toner is applied to the development device, and a voltage (also referred to as a transfer bias) for transferring an image on a recording paper is applied to the transfer roller.
As respective voltages used for these image forming operations, high voltages are used. For example, in order to achieve a good transfer, the transfer bias used is about 3 kV at maximum. An image forming apparatus includes a plurality of power supply devices for high-voltage output.
For generating a high voltage for an image forming operation, as a power supply device for such a high-voltage output, conventionally a wire-wound electromagnetic transformer has been used. The electromagnetic transformer is composed of a copper wire, a bobbin, and a magnetic core, and if it is used for the image forming apparatus, a leakage current from the electromagnetic transformer when outputting the high-voltage is to be minimized.
For this purpose, such a configuration that a winding of the electromagnetic transformer be insulated by a mold or the like is used. Further, since the number of the windings increases according to an electric power (voltage) to be supplied, a size of the electromagnetic transformer becomes larger when a higher voltage is output. Therefore, further reductions in size and weight of the power supply device could not be easily realized.
Thus, in order to further achieve reductions in size and weight of the power supply device, a power supply device, which generates a high voltage using a thin, light-weight, high-voltage output piezoelectric element (piezoelectric transformer) is discussed, for example, in Japanese Patent Application Laid-Open No. 11-206113. More specifically, by constructing a piezoelectric transformer using a thin and light-weight piezoelectric element made of a ceramic as raw material, it becomes possible to generate a high voltage at a higher efficiency than that of the electromagnetic transformer.
Since it becomes possible to keep a distance between the electrodes on the primary and secondary sides of the power supply device by using the piezoelectric transformer, executing special molding work for the purpose of insulation like the electromagnetic transformer may be eliminated. Therefore, if the piezoelectric transformer is used, the reductions of size and weight of the power supply device can be realized. The power supply device, which outputs a high voltage using the piezoelectric transformer, can be applied to various apparatuses that use high voltages, without being limited to such an image forming apparatus as described above.
An example of a circuit of a high-voltage power supply device for high-voltage output using the piezoelectric transformer (hereinafter, also referred to as a piezoelectric transformer type high-voltage power supply device) will be described referring to FIG. 17. A circuit example illustrated in FIG. 17 is, as an example, a charging bias output circuit for outputting a negative bias.
In FIG. 17, the circuit includes a piezoelectric transformer 101Y of a high-voltage power supply. The output of the piezoelectric transformer 101Y is rectified and smoothened to a negative voltage by diodes 102Y, 103Y and a high-voltage capacitor 104Y, and supplied from an output terminal 116Y to a charging roller (i.e., a load, not illustrated).
The output voltage is divided by resistors 105Y, 106Y, and 107Y, and input to a non-inverting input terminal (+terminal) of an operational amplifier 109Y via a protective resistor 108Y. To an inverting input terminal (−terminal) of the operational amplifier 109Y, a control signal (Vcont signal) for the high-voltage power supply, which is an analog signal, is input from a controller (i.e., control unit, not illustrated) via a connection terminal 118Y and a resistor 114Y.
By constituting an integrating circuit by the operational amplifier 109Y, the resistor 114Y, and a capacitor 113Y, a control signal (Vcont signal) smoothened according to an integration time constant determined by the values of the resistor and the capacitor is input to the operational amplifier 109Y. An output terminal of the operational amplifier 109Y is connected to a voltage-controlled oscillator (VCO) 110Y, and the output terminal is connected to a field effect transistor (FET) 111Y connected to an inductor-capacitor (LC) parallel resonance circuit formed by the inductor 112Y and the capacitor 115Y.
The voltage-controlled oscillator (VCO) 110Y outputs a driving signal for switching the FET 111Y. The FET 111Y is switched according to the driving signal. The voltage-controlled oscillator (VCO) 110Y is used to perform such an operation as to raise an output frequency when the input voltage rises, and to reduce the output frequency when the input voltage falls.
Therefore, a frequency depending on an input level thereof is output from the voltage-controlled oscillator (VCO) 110Y. The output signal of the voltage-controlled oscillator (VCO) 110Y drives the inductor-capacitor (LC) resonance circuit, and thereby finally a power supply depending on the control signal (Vcont signal) is supplied to the primary side of the piezoelectric transformer 101Y.
FIG. 18 illustrates a characteristic of an output voltage with respect to a driving frequency of the piezoelectric transformer 101Y. As illustrated in FIG. 18, a characteristic of the piezoelectric transformer is that an output voltage generally peaks at a resonance frequency f0, and it can be seen that a control of the output voltage based on a driving frequency is possible.
Let fx represent a driving frequency when a target output-voltage Edc is output. The voltage-controlled oscillator (VCO) 110Y operates such that the driving frequency varies according to the control signal (Vcont signal). In order to control the output voltage Edc to a higher voltage, the piezoelectric transformer is driven at a driving frequency lower than fx. On the other hand, in order to control the output voltage Edc to a lower voltage, the piezoelectric transformer is driven at a driving frequency far higher than fx.
In other words, an example of the high-voltage driving circuit illustrated in FIG. 17 is a negative feedback control circuit, in which the output voltage is controlled to be a constant-voltage so that the output voltage becomes equal to a voltage determined by the voltage of the control signal (Vcont) input to an inverting input terminal (−terminal) of the operational amplifier 109Y.
As for a circuit control operation, a sweep operation is performed from an initial frequency of a certain high-frequency band to start driving of the piezoelectric transformer, toward fx in a lower frequency direction, until an output coincides with the target voltage value, i.e., Edc value by a feedback circuit. As a result of the sweep operation, upon converging to the frequency fx, the circuit becomes an equilibrium state, and the output voltage Edc will be stably output.
However, there is the following situation in the above-described piezoelectric transformer type high-voltage power supply device. In the circuit configuration illustrated in FIG. 17, a driving signal 300 illustrated in FIG. 19 is input to the gate of the switching element 111Y to drive the inductor-capacitor (LC) resonance circuit. As a result, a signal in a flyback waveform as illustrated in a signal 301 is input to a piezoelectric element.
It is known that undesired resonance frequencies (hereinafter, referred to as spurious frequencies) indicated by fsp1, fsp2, fsp3, fsp4 in FIG. 18 appear by applying an electrical oscillation on the piezoelectric element in a square-shaped waveform different from such a sinusoidal wave. The spurious frequency is dependent on the structure of the piezoelectric transformer such as a width and a thickness.
FIG. 20 illustrates an input waveform of the piezoelectric transformer generated by the LC resonance circuit when a switching operation is performed at a frequency in the neighborhood of the spurious frequency fsp1. FIG. 20 includes, similar to FIG. 19, a gate signal 300 of the switching FET, and a drain voltage 301 of the switching FET, i.e., an input voltage waveform of the piezoelectric transformer. In addition, FIG. 20 includes a drain current 302 of the switching FET.
As illustrated as the drain voltage 301 in FIG. 20, distortions are produced in the voltage waveform by resonance at the spurious frequencies of the piezoelectric transformer separate from resonance frequency determined by values of L and C. As a result, at timing when the drain voltage of the switching FET is not “0”, the gate is turned on. Thereby, as illustrated in the waveform 302, an excessive drain current will eventually flow.
As for a circuit operation as described above, the sweep operation is performed from an initial frequency to start driving of the piezoelectric transformer, to a lower side of frequencies so as to reach a predetermined voltage value. Therefore, at the time of starting, the sweep operation causes the frequencies to pass through the spurious frequencies. Each time, at the time of the starting, an excessive current transiently flows in the switching FET, as illustrated in FIG. 20.
In other words, elements that can withstand such an excessive current as the switching FET are selected, and thus users cannot select more inexpensive elements as the switching FET.
In order not to generate the transient excessive current, it is conceivable to set a control range of the output voltage of the piezoelectric transformer to a frequency range in which there is no influence of the spurious frequencies (in the range lower than a frequency corresponding to a voltage Edc′ of FIG. 18). However, since this makes a voltage control range narrow, it cannot be an adequate measure. For example, when a much lower voltage is used, the output voltage will be subjected to the influence of spurious frequencies.